Roentgen discovered X-radiation by the inadvertent exposure of a silver halide photographic element. In 1913, Eastman Kodak Company introduced its first product specifically intended to be exposed by X-radiation (X-rays). Silver halide radiographic films account for the overwhelming majority of medical diagnostic images. It was recognized almost immediately that the high energy ionizing X-rays are potentially harmful, and ways were sought to avoid high levels of patient exposure. Radiographic films provide viewable silver images upon imagewise exposure followed by rapid access processing.
One approach, still in wide-spread use is to coat the silver halide emulsions useful in radiographic films on both sides of the film support. Thus, the number of X-rays that can be absorbed and used for imaging are doubled, providing higher sensitivity. Dual-coated radiographic films are sold by Eastman Kodak Company as DUPLITIZED.TM. films. Films that rely entirely upon X-radiation absorption for image capture are referred to in the art as "direct" radiographic films while those that rely on intensifying screen light emission are referred to as "indirect" radiographic films. Because the silver halide emulsions are used to capture the X-rays directly, the silver coating coverages of direct radiographic elements are generally higher than for indirect radiographic elements.
Among the "direct" radiographic films, are films most commonly used for dental intra-oral diagnostic imaging and hereafter referred to as dental films. Intra-oral dental imaging presents obvious barriers to the use of intensifying screens. Thus, dental films utilize the coated silver halide to absorb X-rays.
There are other applications for direct radiographic films, such as in various industrial applications where X-rays are captured in imaging, but intensifying screens cannot be used for some reason.
U.S. Pat. No. 5,370,977 (Zietlow) describes dental films having improved characteristics and containing certain tabular grain silver halide emulsions. No spectral sensitization is used in such dental films, but in order to avoid fogging the films with inadvertent light exposure, the emulsions contain what is identified as a "desensitizer" that reduces emulsion sensitivity to light. Conventional processing solutions and conditions are described for these dental films.
Other desensitizing compounds for radiographic films are described in U.S. Pat. No. 3,630,744 (Thiers et al) for reducing film sensitivity to roomlight and UV radiation. Conventional processing of these films is also described.
It is the prevailing practice to process direct radiographic films for more than 3 minutes because of higher silver coverage. Such processes typically include black-and-white development, fixing, washing and drying. Films processed in this manner are then ready for viewing.
Photographic developing solutions containing a silver halide developing agent are well known in the photographic art for reducing silver halide grains containing a latent image to yield a developed photographic image. Many useful developing agents are known in the art, with hydroquinone and similar dihydroxybenzene compounds and ascorbic acid (and derivatives) being some of the most common. Such solutions generally contain other components such as sulfites as antioxidants, buffers, antifoggants, halides and hardeners. A workable pH for such solutions is usually in the range of from about 10 to about 11, depending upon the developing agent and other solution components.
Fixing solutions for radiographic films are also well known and include one or more fixing agents, of which thiosulfates are most common. Such solutions also generally include sulfites as antioxidants, and hardeners (such as aluminum salts), and a buffer (such as acetate), and have a functional pH range of from about 4 to about 5.5.
"Monobath" solutions are also known in the art of photographic chemical processing. Such solutions generally require long processing times and contain chemical components common to black-and-white developing and fixing solutions. They also typically have an alkaline pH and contain a sulfite.
Dual-coated indirect radiographic elements described in U.S. Pat. No. 4,803,150 (Dickerson et al) contain microcrystalline particulate dyes that reduce "crossover". These elements are designed for use with intensifying screens. Crossover occurs when some light emitted by the screen passes through the film support and exposes silver halide grains on the opposite side, resulting in reduced image sharpness. The noted particulate dyes absorb unwanted actinic radiation, but are decolorized during conventional processing. Thus, a pH 10 developing solution is described for its conventional use as well as to decolorize the dyes within 90 seconds. Conventional fixing and washing follow.
Using conventional processing technology, such particulate dyes that allow roomlight handling would be rendered ineffective, since the development step is carried out at high pH in the presence of a sulfite. Thus, in a conventional multi-step process, the processed films cannot be handled in roomlight between the developing and fixing steps. Conventional "monobath" solutions do not allow for sufficient development since both exposed and unexposed silver halide is indiscriminately removed by the fixing agents, especially at the long processing times employed with these solutions.
Direct radiographic films, including dental films, thus have some sensitivity to roomlight and UV as well as X-rays, and therefore care must be taken to avoid inadvertent room-light exposure before and during processing. There has been a desire for radiographic films that are less sensitive to roomlight, and that can be handled and processed without the need for a darkroom or other special conditions. Such films would have a number of useful applications, such as dental and industrial imaging. However, conventional processing solutions and methods cannot be used to provide suitable radiographic images in such films.
Copending and commonly assigned U.S. Ser. No. 08/956,305, filed Oct. 22, 1997, describes the use of separate developing and fixing compositions for processing roomlight handleable films, including radiographic dental films in sequential processing steps. While those compositions represent an advance in the art, they must be separately balanced in pH in relation to each other so that the light protecting dyes and desensitizers are not deactivated prematurely.
Using current processing technology, the dyes that allow roomlight handling would be rendered ineffective, since the development step is carried out at a high pH in the presence of sulfite ions. Thus, in a conventional multi-step process, the films could be handled in roomlight between the development and fixing steps. Conventional monobath processing solutions do not allow for sufficient development, since exposed and unexposed silver halide is indiscriminately removed by fixing agents, especially at the long processing times employed using those solutions.
In copending and commonly assigned U.S. Ser. No. 08/970,869, filed Nov. 14, 1997, by Fitterman et al, processing of roomlight handleable photographic elements is described whereby two-stage processing (development and development/fixing) is carried out in a single light and fluid-tight processing container. Thus, while the elements can be handled in roomlight, they must be processed in the dark.
U.S. Ser. No. 09/046,447, noted above, describes "opaque" two-stage processing using a developing step followed by addition of fixing agents to provide a developing/fixing solution for the second step. Opacity is provided using particulate opacifying agents, such as carbon black. Such dilute carbon black compositions adequately block light during processing and remain sufficiently dispersed to process black-and-white films within 60 seconds.
While this processing system is effective for providing black-and-white images within 60 seconds, it has a disadvantage in that the particulate opacifying agent (for example, carbon black) is difficult to completely remove from the surfaces of the processed films. In addition, because of the opaque nature of the processing compositions, it is difficult to observe the progress of development by looking at the solutions.
Therefore, a technology is needed wherein black-and-white photographic elements can be both handled and processed completely in roomlight, with the attendant advantages of known processing systems, but which avoids the problems noted above from the use of carbon black or other opacifying agents.